kernel/irq: fix sparse warning: make symbol static

While being at it make every occurrence of 'do_irq_select_affinity'
have the same signature in terms of signedness of the first argument.

Fix this sparse warning:
  kernel/irq/manage.c:112:5: warning: symbol 'do_irq_select_affinity' was not declared. Should it be static?

Also rename do_irq_select_affinity() to setup_affinity() - shorter name
and clearer naming.

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Acked-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Ingo Molnar <mingo@elte.hu>